Look to your scope as others said. 4064 is like most other powders and can exhibit sensitivity to temp change. Federal still uses a similar powder in the FGMM stuff for .308. Velocity difference can be as much as 1 fps per 1 degree. So that could be a 30-40fps shift. If your on the edge of a node that could easily push you out. You mentioned you had h4350. That is by far the most popular powder for the .260 with bullets over 120 gr. maybe a powder switch is in your future. Hope you find the issue.